Shortenable raisable landing gear is known that includes a leg hinged to the structure of an airplane, the leg being constituted by a strut housing a shock absorbing mainly composed by a sliding rod and a shock absorber plunger rod, together with a linkage connecting said plunger rod to the strut and under the control of a resilient connecting rod having a threshold and fixed to the airplane structure so as to pull the shock absorber while the leg is being raised and so as to push said shock absorber while said leg is being lowered.
That disposition makes it possible to obtain landing gear which is long in the undercarriage-down position and short in the undercarriage-up position (thus making it easier to integrate the landing gear inside the associated housing in the airplane).
Another possible approach for obtaining the same result consists in using extensible landing gear of a structure that is capable, while the airplane is stationary on the ground, of exerting a force equivalent to the static load thereof, thereby raising the strut of the shock absorber relative to the sliding rod.
The first approach, which is the approach adopted by the present invention, consists in using landing gear of the above-specified type having a shortenable leg, in which the linkage is organized to lengthen the leg while the landing gear is being lowered, before the wheels touch the ground: lengthening is then intrinsically controlled by the structure without the pilot being required to trigger any kind of mechanism, in contrast to extensible landing gear.
FIGS. 1 and 2 show prior art landing gear having a shortenable leg, and complying with the above definition.
The axial section of FIG. 1 thus shows raisable landing gear 100 comprising a leg 101 hinged about an axis 102 to the structure of an airplane, and including a strut 103 and a sliding rod 104 fitted with a shock-absorbing plunger rod 105 disposed coaxially about the axis X of the strut. The bottom of the strut 103 has forks 106 hinged to two actuators 107 for steering the wheels, the rods of said actuators being connected to a rotary sleeve 109 coaxial with the strut. The bottom of the sliding rod 104 carries a set of wheels (represented solely by an axle 109), and it is connected to the strut 103 by a scissors linkage having two arms 110 and 111. The sliding rod 104 also includes a transverse wall constituting an intermediate partition that defines a bottom hydraulic fluid chamber 113 which communicates in turn via a diaphragm with a top hydraulic chamber 114 inside the plunger rod 105. The bottom of the plunger rod 105 has an end 115 fitted with holes 116 (to perform the above-mentioned diaphragm function), and has a rod 117 secured to the partition 112 of the sliding rod 104 passing through the center thereof. A system 118 of corresponding helical cams secured respectively to the sliding rod 104 and to the plunger rod 105 provide recentering and wedging in a particular relative angular disposition.
A linkage 130 serves to pull the shock absorber without compressing it when the plunger rod 105 is pulled (towards the hinge axis of the leg) while the landing gear leg is being raised, or to lengthen the leg while the plunger rod 105 is pushed back by said linkage (while the landing gear leg is being lowered) as represented by the position shown in FIG. 1. The plunger rod 105 slides in a bearing 119 formed in the upper portion of the strut 103, and its top end 120 is connected to the linkage 130.
The linkage 130 includes an arm 121 hinged at its bottom end to the plunger rod 105, and at its top end to one end of a lever 122 which is itself hinged at 123 to a pin carried by gusset plates 124 on the strut. The other end of the lever 122 is hinged to a resilient connecting rod having a threshold (a "spring rod") 125 which is in turn hinged to the structure 126 of the airplane. The locked position of the linkage 130 is provided by the resilient connecting rod 125 having a threshold keeping the two alignment arms in an abutment position (by means of a lug 127 associated with the arm 121) which is slightly out of alignment, being slightly beyond dead center from the side that closes up when the undercarriage is raised.
FIG. 2 is on a larger scale and shows the top portion of the above-mentioned landing gear 100, and in particular it shows said linkage, using continuous lines to show the position that corresponds to the leg being lengthened and the undercarriage down, and chain-dotted lines to show the position corresponding to the leg being shortened with the undercarriage up, with shortening being intrinsic to the linkage. The lengthening or shortening stroke obtained is referenced c.
Such landing gear gives full satisfaction under normal operating conditions. However, if the resilient connecting rod having a threshold should fail, then normal operation is affected.
Thus, if the connecting rod breaks while the landing gear is in its raised position or while it is being lowered, then the lowering of the landing gear is affected insofar as the leg does not lengthen normally. In addition, in the undercarriage-down position, there is no longer any positive locking of the alignment in the linkage, and said alignment can thus collapse on impact when the wheels touch the ground, which would cause the landing gear to collapse vertically.